I think I spawned with debug mode enabled
For a long time I thought this was the default for the vanilla client, but after hearing some people who mess around with resource packs talking about some of these things, I went down a research side-quest on the community-maintained wiki and it turns out these aren't default and there's a debug mode. It's purely client-side, so nothing bannable or anything, but the overlay is useful at times and the scene inspector makes it easier to learn things. It's made of several variants of the [Synesthesia] status effect combined with the [Ideasthesia] effect, and it lets me open secondary windows that show things like previously seen wiki entries, known questlines, a zoomable minimap and scene inspector (albeit limited to what the client has loaded), and even utilities like the physics simulator and block-based logic programming editor, all shown in a weird language that doesn't exist but I can understand. Even more interesting, the visual overlay always shows things like subtitles, geometric relations, projected corner angles, and trajectories, with a way to import them into the physics simulator. It tends to add very faint static on top of everything, though, and when the brightness is really low, sometimes it starts showing random fractals and sometimes faint objects. There's also this weird background window that's always open that shows colors and patterns based on music and sound effects.